The present disclosure relates to fabricating and making interconnections of transducers and transducer arrays, particularly to fabricating and making interconnections of capacitive micromachined ultrasonic transducer (cMUT) arrays.
Making interconnections of a transducer array on a proper substrate is a key for any application using a transducer array, especially for a transducer array with large number of elements. One such example is ultrasound imaging which uses an ultrasound transducer arrays. The proper packaging and interconnections of the ultrasound transducer array is very important to achieve desired performance with lower cost.
Capacitive micromachined ultrasonic transducers (cMUTs) are electrostatic actuators/transducers, which are widely used in various applications. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. Ultrasonic transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and numerous other practical applications. A typical structure of a cMUT is a parallel plate capacitor with a rigid bottom electrode and a movable top electrode residing on or within a flexible membrane, which is used to transmit/accurate (TX) or receive/detect (RX) an acoustic wave in an adjacent medium. A direct current (DC) bias voltage may be applied between the electrodes to deflect the membrane to an optimum position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an alternating current (AC) signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane in order to deliver acoustic energy into the medium surrounding the cMUT. During reception an impinging acoustic wave causes the membrane to vibrate, thus altering the capacitance between the two electrodes.
The top electrode is typically the movable electrode of the cMUT used to transmit and detect the acoustic wave. The movable electrode interfaces with the medium and usually is on the top of the substrate surface, thus called the top electrode. The bottom electrode is typically at least partially fixed (static) and usually locates underneath the top electrode in the substrate. In most cMUT applications, a cMUT array having multiple cMUT elements is used to perform a desired function. Usually each cMUT element in the array is addressed (connected to a signal line) from one of its two electrodes and another electrode is connected to a common electrode shared by multiple cMUT elements or all cMUT elements in the array. Currently, most cMUT arrays, especially 1-D arrays, have common fixed bottom electrodes. Each element in the array is individually addressed from its moving top electrode. The reason for using the top moving electrode as the individually addressed electrode is simply the ease of fabrication.
However, since the moving top electrode is used to interface the medium, there are some limitations to the methods of making electrode connections to the top electrode. For performance, packaging and electrical connection consideration, it is often desired to make the fixed bottom electrodes of a cMUT array the hot electrode (i.e., the individually addressed electrode), and make the top electrode the common electrode.
There are a few methods of processing, packaging and making interconnections known to make the bottom fixed electrodes as the individually addressed electrodes. One typical method is to drill holes on the substrate using various methods, and subsequently fill the holes using a conductive material. The process is relatively complex and has performance limitations. Also there is a trade-off between the parasitic capacitance and the conductivity of the interconnection in this method. This trade-off affects the device performance. Moreover, this method usually cannot make a flexible cMUT array.
Another method is to cut through the substrate underneath the elements to form individual bottom electrodes. The cut-through can be done with etching the substrate material. However, since the substrate underneath each element in the array is completely separated from its neighbor elements or the rest of the substrate, these methods require techniques to support or hold the elements in the array together during the cutting through of the substrate. One typical technique is to bond the cMUT with its fabrication substrate to another substrate, which provides support or holds the transducer elements in the array to continue the fabrication process, making interconnections and packaging. Another technique is to hold the transducer elements using a filler material between elements during the process. The material is usually an insulation material and can be a dialectical material or combination of multiple materials.
The existing methods are mainly developed for making interconnections of a 2-D array. These methods are relatively complex and may not be cost-efficient for 1-D array device. A better interconnection method for a cMUT array, especially for a 1-D cMUT array, is desired.